w \t 11 1 I i , s , \ i . :i J / \ i 41 \ ^4^ ■»r tf . • * i *3 » if '- / - ■ * i - y 4 ■ ^■w- r^ % >*■•' Xkit' d ■ » • $■ STRUCTURAL DESIGN OF REFRACTORY CONCRETE BY R. T. GILES The Atlas Lumnite Cement Co New York, N. Y. Structural Design of Refractory Concrete By R. T. GILES The Atlas Lumnite Cement Co New York, N. Y. REFRACTORY concrete differs ■^ from structural concrete in that a calcium aluminate cement and refractory aggregates are used instead of portland cement, sand and gravel or stone commonly used in structural concrete. Refractory concrete is used for continuous ex- posure to temperature as high as 3000 deg. F. while structural con- crete is usually limited to continu- ous exposure of temperatures no higher than 500 deg. F. to 600 deg. F. Very little is known about struc- tural design of refractory concrete at this time. Only recently have data been available on the ultimate unit strength of the concrete after it has been exposed to elevated temperatures over long periods of •"Inyestigation of Certain Prop- er' of Uefractory Concrete," Bul- letin, American Ceramic Society, Sept. 1939. time.* Plots of these data are shown in Figs. 1 and 2. Refrac- tory concrete heated only on one side differs from structural con- crete, in that at temperatures above 1500 deg. F. it has a higher strength at the inside face and outside face than it has in the middle of the mass. The strength at the inside face is a result of its exposure to elevated temperatures. The strength at the outside face exposed to the air is similar to that of regular concrete. At some point in the inner part of the con- crete there is a weak section or plane similar to a very low quality concrete. To use any one of these strengths individually as a unit design strength would be mislead- ing. It is necessary, therefore, to use the average strength of the concrete for design purposes. Fig. 3 shows the strengths of the concrete which can be expected if a temperature gradient through c ■ <n 2000 1800 1600 1400 1200 10OO 1 1 r~ Dotted Line . Clay Added to High Vitrification Cloy Aggregates Solid Line = High Vitrification Clay Aggregatea. T 1 1 1 1 1 1 1 1 r Low Vitrification Clay Aggregate or 1 1 1 1 Low Vitrification 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Temperature, Degree* F. Fig. 1 The average ultimate strength of concrete slabs or walls exposed to elevated temperatures for long periods. These results are applicable to concrete with cold compressive strength of 2000 lb. per sq. In. or more and with cold Rexural strength of 500 lb. per sq. in. or more when using calcium aluminate cement as the binder and a strong aggregate in the proportions of 1 bag to 4 cu. ft. of aggregate. 1000 C 800 200 400 600 800 1000 1200 1400 Temperature, Degrees F. 1600 1800 2000 Fin 2 Ultimate compressive and flexural strength which may be used for determining the average ultima UsLg of concrete slabs or walls exposed to elevated temperatures over long period of ^ time. ThKP results are aoolicable to concrete w th cold compressive strengths of 1000 lb. per sq. in. or less when using a calcium aluminate cement and a weak insulating aggregate in the proport.ons of one bag of cement to 4 cu. ft. of aggregate. the wall is plotted and strength of each section used to calculate the average unit strength. Example: Using a furnace wall of 6" thick exposed to 2100 deg. F. on the hot face with 300 deg. F. on the cold face. This gives a temperature drop of 1800 deg. F. or 600 deg. F. for each 2" section. The average temperature in the section containing the hot face is 1800 deg. F. The average temper- ature of the middle section is 1200 deg. F. and the average tempera- ture of the section containing the cold section is 600 deg. F. By re- ferring to the design chart, Fig. 1, we find the compressive strength using high vitrification aggregate at 1800 deg. F. to be 500 lbs. per compressive strength F. to be 600 lbs. per compressive strength F. to be 1200 lbs. per sq. at sq. in. ; the 1200 deg. in.; the 600 deg. at sq. in. By adding 1200 + 600 + 500 and dividing by 3, the number of sections, we have 766 lbs. per sq. in. for the average compressive strength for the concrete in the wall. Example: Using a 12" wall ex- posed to 2700 deg. F. on the hot face with a cold face temperature of 300 deg. F. (See Fig. 3) This give- a temperature drop of 2400 deg. F. or 400 deg. F. for each 2" section. The average temperature in the first J section is 2500 deg. F. which has lbs. per sq. in. com- pressive strength. The next sec- tion has an average temperature of 2100 deg. F. which has a com- pressive strength of 600 lbs. per sq. in. The next section has an average temperature of 1700 deg. F. which has a compressive strength of 450 lbs. per sq. in.; the next a temperature of 1300 deg. F. and a compressive strength of 500 lbs. per sq. in. ; the next a tem- perature of 900 deg. F. with a compressive strength of 850 lbs. per sq. in., and the outside section an average temperature of "500 deg. F. with a compressive strength of 1350 lbs. per sq. in. By adding + 600 + 450+500 + 850+1350, and dividing by 6, we have 625 as the average compressive strength in lbs. per sq. in. for the 12" thick- ness. Of course, it is realized that the average strength is not an exact measure of the effective strength of a wall whose elements have varying unit strengths. Formulas for exact strength can be integral ed by means of the Calculus, but their application is laborious. In addition, consideration should be given to any difference in modulus of elasticity of the concrete at dif- ferent temperatures. Assuming that these moduli do not differ ap- preciably, a simple approximation, 4 Temperature, Degrees F 3000 2800 o o to o o o o o o o o H o o to r-t o o to o o l-i o o H O o CM O O CO CM O O lO CM O O fc- CM o •H w CO (D ft E o o 2600 2400 2200 2000 1800 1600 Temperature Gradient of 12" Wall oo 1400 0) s 1200 1000 Strength Gradient from Design Chart No. 1 Arbitrary Strength Gradient 800 600 400 Cold Face 300 Dee. F. 2 4 11 Inche s Fig. 3 Wall strength of refractory concrete. Broken line: Strength gradient of refractory concrete wall 12 in. thick exposed to 2700 (leg. on hot face; outside surface assumed to be 300 deg. F. bofid line: Arbitrary straight line strength. From this the following formula is taken: Effective Structural Strength in Compression for each linear inch of wall equals 800 times each inch of wall thickness exposed to temperature of 2300 deg. F. and less. that will cover the most unfavor- able conditions ordinarily encoun- tered, is feasible by assuming a wall whose unit strength increases uniformly from zero on the hot side to a maximum on the cool side. Considered as a column, its effective moment of inertia, and therefore its strength, is theoreti- cally two-thirds of that of a wall in which an average strength is assumed to be uniformly distrib- uted. Hence, if the value found by the method of averages is reduced by one-third, a strength is determined that is believed to be always on the safe side. This will represent the ultimate strength, to which a suitable factor of safety should be applied. 5 800 00 1200 1400 1600 ieoo i xv o.oatf ««g« ( !• '1 0,01 m4m ■ Volume CK*nfti , I ; will ikftJM tl IliHU U>K ■ ra ■ ■ ' bi *, Ml tl i» i-uMo • » 1 I • ■ 1 ad thai tl . . . I all (awn ■ - | W I * t nn 'iMt I a | » ttf* «f I W«d b> v ead tlat m OBJ it rnj»<l« w ith r. ♦ - ... t iry ■ftrcMJ b- ic k All d I I i £rz^^^~:v:::rr::":q Section No# 1 4- i Sectlon No. 2 \ CO t Section No. 3 I _l 2 o G O Y Section Ho* 4 _L Section No # 5 t :=r o H Beam Relnf orclngv_ jj Roof Slab Fig. 5 Refractory concrete flat arch for precast or cast in place: Pour sections 1, 3 and 5; remove forms "A" (See Fig. pour sections 2 and 4. place sections 6) ; paint Procedure for cast-in- with fire' clay slurry; section of arch itself is a rein- forced concrete beam to carry the weight of the arch section. The concrete in the flat portion of the section is composed of the cement and an aggregate of low conduc- tivity. The concrete in the beam surrounding the reinforcing bars is composed of the cement and an aggregate of higher conductivity. The concrete in the flat portion has a low heat conductivity as a result of the use of the insulating aggre- gate, while the concrete in the beam has a higher heat conductiv- ity as a result of the use of dense aggregates. The higher conduc- tivity concrete in the beam and the lower conductivity concrete in the slab each has an important bear- ing on the temperature of the re- inforcing steel. Stated otherwise, 7 Wood For« ■&^ Fig. 6 Diagratnma view of forms and .onforclnu. hao In place. This method used in con ructtng flat arch shown In Fig. 5 the diffi in tin onducth I etes has an in. beaiing on 1 " .uireri t) s Of t) I lab to mail ■ I nj, in tl ing high i 6 )'• While this method ha been l; plied to flat r Lab plied to erl al > floor - as well. Tbl BUel hod limited to t*-mp< ares i wj h satisfa insulating a«- g I i tail -• fad coats, 1; '»f no • '« or units f hijihh *■ frartory mat* . uee'i 1 protect th< insulal rig cte. The fir a h or wail is then serviceable at mud high< i temp, iturcs than is the unproi d insulating concrete i • t, k. and 9 -how the thick- i,. • t tin- slah using in slating %ggr« eatei f concrete with ;i 1 f, r of 2, 8 and 4 when u 1 with a COIN • ' 1< in th< beam w ilh K f. toi in ' si< slating ■ I the - () f i i lah u - I \y>. 7, 8 and 9 Cor reinforced beam on , v i he n infoi cing . not n B 1 D 2" D the e>; f i}< l as shown ii I Y I. th< I show. K rtej rnurh In; than 1 f >, it 1" re s f , ■ ^ si Q 8 S3B 2500 o lO (1) Reinforced Concrete 3" or 4" Thick* K As Shown on Graph* (2) Insulating Concrete, K 2. Thickness As Shown on Graph. Temperature at Joint 600 Deg. F # (4) Temperature of Hot Face as Shown on Graph lO m cc o H t~i H WWW CD W • 2000 © ■P O U c Q B © © o a ■P o 1500 1000 500 Thickness of Insulating Concrete, Inches 7 Thickness of slab using insulating aggregates for concrete with a l< factor of 2, when used with a concrete in the beam with higher l< factors. so far been demonstrated than an economical concrete can be devel- oped which will have a much high- er K factor. Indications are that at temperatures of 600 deg. F. and lower, the use of olivine or magne- site offers the greatest promise but that the most that can be hoped for from either of these aggregates used in concrete is to develop a K factor of approximately 15. For concrete made of trap rock a K factor of 8 may be assumed, while with aggregate made from old fire brick a K factor of only 6 can safely be used. It has not been definitely proven that a temperature somewhat high- er than 600 deg. F. could not be used for ordinary carbon steel. However, it is felt that this tem- perature is conservative and until such time as experience shows it to be too conservative it should be used. Application to High Temperature Furnaces Refractory concrete will un- doubtedly be used in furnaces, the temperatures of which are entirely 9 - ^ ♦ I II I 1 9 I I t » on Ortph. « * } TMctot » At ShOTG o» Graph. "tap** • • * * •§ ••' 4 ipiril ■ Of let Fie# Ai ■ : S -• « i « m l l« ' 0»C» « < <u itffit'eli » *' . - 1* • d if .«r<«|pr I ' CM m if ' »lt-»jtft *'> • (** A*z<l .'Bi n ) * * _ *. * < Insulating Concrete 3" or 4" Thick. K As Shown on Graph. Insulating Concrete. K 4. Thickness As Shown on Graph. Temperature at Joint 600 Deg . P. (4) Temperature of Hot Pace A a Shown on Graph 2 500 . 2000 u- 2 t 1500 8 m u I I c ** 1000 c o 500 Thickness of Inaulattn* Concr.t., Ioch«* . Fin 9 Thlchnui of a slab tiling nrj aggregate* for concrete with a K fictor of 4 when usnl with .1 concrete in the beam with higher K factor. ..I Mr '"' P^'d in pi of the ••Mi .1 bar supports. In this method conditio are ither ideal; whereas, when ren remg 1 i- embedded In the mcrete, the expansion of the steel and the shrinkage of the concrete whi-n exposed to elevated temper ires, theorvt i illy at bast, tend • break down the concrete. The method shown in Fig. to is much mure compatible with the volume change of thi wo material - when heated to elevated temperatun It noted that the shrinkage of the concrete, to some extent at leas mipensatea for the expan »n of the steel. While no installatioi D ring the method shown in Fig. 10 h e been made at the present time of * t- inp. 1 ... 11 shows ■ full floating flat arch 4 in. thick and 8 f square, which is the principle util- 11 If % ^ m hm»lnc ror«t# In PUw — -1 fe» - • ■ • Ami On r I . « *"'' ' #• m < J • . roil • • I . r r I' 1 '* I ■M ' i MihH ized for this method of construc- tion. Fig. 12 also illustrates a method of construction for precast or cast- in-place sections. This method of- fers a simple and flexible method which can be used for large sec- tions. Figs. 15 and 16 illustrate the method of reinforcing shown dia- grammatically in Fig. 12, except that there were no cross angles installed. While the information given relative to the amount of steel buried in the concrete and the amount exposed to the air is thought conservative, it should be borne in mind that more experi- ence will be needed to lay down any hard and fast rules. The question of sprung arches is relatively simple. They may be cast in place or precast in sections and lifted into place with a crane. Sections of arches with a 14'0" span, weighing 3 tons with no reinforcing, have been precast and set in place with a crane. Where doubt exists about the strength of the concrete in an arch section it can be reinforced with angle irons on the outside corners. When so used one flange of the angle is ex- posed to the air for rapid dissipa- tion of the heat to maintain a low temperature in the steel. The an- les must be bent to conform to the radius of the arch and fas- tened together at the ends and several points in the middle. With reference to Figs. 13 and 16, this method may also be ap- plied to flat arches and wall sec- tions. It is doubtful if in many cases it will be desirable to reinforce the Fig. 11 No installations have been made using the reinforcing method similar to that shown in Fig. 10. The above illustration shows a full float- ing flat arch 4 in. thick and S ft. square, which is the principle utilized for this method of con- struction. vertical or side walls of a furnace. However, in Fig. 14 reinforced side walls are shown. (See also Fig. 17). One side shows bars embedded in the concrete, while the other shows angle irons with one flange embedded in the con- crete and the other flange exposed to the air to facilitate the dissipa- tion of heat and maintain a low temperature in the angle. It is obvious that the methods of con- struction can be interchanged to a considerable extent. For example, the methods shown for reinforcing the side walls can be used for the bottom. It is important to remem- ber that the temperature of the steel should not exceed 600 deg. F. in any case. The bottom in this sketch shows a method of construc- tion similar to that shown in the roof arch, Figs. 5 and 6. The roof shows a method of construction similar to that shown in Fig. 10. 13 Flange of Angle In Concrete Alternately Bent In Opposite Directions As Shown A Outside Angle Cupped In Slightly \ Section "AA" Fig. 12 Section with angle iron reinforcing suitable for arches, waJIs and bottoms. While the side walls show the reinforcing in one direction it can be used in the opposite or both directions if desired or supple- mented with wire mesh as shown. When refractory concrete is de- pendent wholly upon its own strength it would appear logical to limit the dimension of any slab to not more than 4'-0" in any one direction. However, when the re- fractory concrete slab is reinforced by other methods it is felt the size of the slab can be increased to any -ize which the particular method will permit. Fig, 10 illustrates a method in which the maximum dimension should be limited. Figs. 5 and 6 illustrate a method in which the maximum dimensions of the slab would be largely dependent upon the strength of the reinforced beam. The side walls in Fig. 14 illustrate a method in which it is felt the dimensions of the concrete 14 might be very large — in fact, with almost no limit. This is also true of the method shown in Fig. 12. Repairs While the method of construc- tion is important there are under certain conditions other considera- tions of equal importance. Under severe conditions where repairs are frequent and inevitable, the means for making replacements is of equal if not greater importance. No great difficulty is encountered in repairing the bottom of most furnaces. After the slag is chipped off, another layer of refractory be placed on top of of the concrete slab concrete can that portion remaining. Much the same condition exists with relation to the side walls ex- cept that forms are necessary to form the inside surface while the remaining concrete acts as the out- side form. In large furnaces the side walls may be repaired by shooting a refractory concrete mortar with a cement gun. Roof arches offer a greater prob- lem and it is felt the method shown in Fig. 10 offers the simplest meth- od of repair especially if precast units are used in the original con- struction. For example, if slab No. 2 in Section No. 3 needed re- placement it would only be neces- sary to remove slabs No. 1 and 2, insert a new slab for No. 2 and replace slab No. 1. This repair could in many cases be made with- out serious interference with op- eration and without materially Cross Ties As Heeded 3" Angle Iron Section "AA n 3" Angle See End View % . »* /-?* ^k i/e n -T_l. 3" Angle — -h- Enlarged Detail of Corner Corners of Angles Mltered and Welded End View Fig, 13 Sprung arch section showing angle iron frame 15 "A" = Thickness of Roof Slab "B" & "C» = i "A" Joint ~T ;.OJ ft?.*»^K wO o-r^O-rr^-i • -1 *, 6>*-_ <* *£ J * * 300 Dee. Joint Angles Reinforcing Bars 2100 Dee. F. Mesh Reinforcing Joint 3 r ^ - Mesh Rein- forcing Joint - : . s: Reinforcing Bars Fig. 14 Illustrating several different methods of reinforcing refractory concrete. A different n»e th °d of reinforcing is shown in each side. Where metal reinforcing is embedded in the concrete, it should be so placed that the temperature of the steel will not exceed 600 deg. F. All joints should be given a paint coat of fire clay slurry. dropping the temperature in the furnace. The application of the fire clay slurry to these joints dur- ing construction has an important part in the removal of these sec- tion.-. If a highly refractory clay is used, a plane of weakness at each joint will exist which will greatly facilitate the removal of i dabs. A cement gun can also be used for repairing these slabs but it would prevent the removal of any of these slabs for replacement at a later date. The repair of a roof slab as shown in Figs. 5 and 6 would require the use of a o-ment gun or the removal of a complete section. Under these conditions it is felt this method of construction is best adapted to furnaces which seldom require repairs or replace ment. Where conditions are such that it is desirable to use reinforcing which will attain a temperature higher than the 600 deg. F. maxi- mum for carbon steel, special steels should be used. Size Limitations It is commonly accepted that .rick walls of 4W thickness ar 10 Fig. 15 Example of the method of reinforcing shown in Fig. 12 except there were no cross angles. not suitable for heights greater than about 3 feet, while 9" brick walls should be limited to a height of 8 feet; 13 M$" brick walls to a height of 12 feet, and for greater heights the brick walls should be 18" or more thick. Refractory concrete can be rein- forced so that practically any thickness can be used in walls of almost any height. Assuming that the concrete wall is 80% as thick as the brick wall, approximately the same heat loss will result, while there will be much less heat storage in the concrete. This is explained by the fact that if clay fire brick are crushed to granular sizes, mixed with the cement and cast as concrete, about 15% great- er porosity exists in the concrete than in the brick before being crushed. This increase in porosity results in greater insulating value and decreases the weight per cubic foot which, when combined with the reduction in thickness for the concrete wall, makes its weight only about 67% that of brick wall of the same furnace. Fig. 16 Another example of the method of reinforcing shown in Fig. 12 except there were no cross angles. 17 I Reprinted from Industrial Heatin 1940 V u II f.